The present invention relates to an apparatus and a method for controlling an electric power steering system used in vehicles such as automobiles.
An electric power steering system uses an electric motor to assist movement of a steering wheel. The electric motor generates assist force for reducing steering force of the steering wheel and compensation force for compensating for the movement of the steering wheel.
When the vehicle is driving, if the steering wheel is released after being rotated, the self-aligning torque, which is applied to the tires from the road surface, returns the steering wheel to the neutral position. When the steering wheel is automatically returned to the neutral position, the electric motor receives inertial force. The inertial force adversely affects the returning of the steering wheel to the neutral position. Specifically, when the vehicle is driving at a low speed, the inertial force delays the returning of the steering wheel. When the vehicle is driving at a high speed, the inertial force generates a vibration, which causes the steering wheel to pass the neutral position.
To eliminate such drawbacks when the steering wheel is automatically returned to the neutral position, the electric motor generates the compensation force for assisting the steering wheel to return to the neutral position when the vehicle is driving at a low speed. While the vehicle is driving at a high speed, the electric motor generates the compensation force to improve the convergence of the steering wheel to the neutral position when the steering wheel automatically returns to the neutral position. Further, the electric motor generates the compensation force to compensate the load generated due to friction in the transmission path of the steering force between the steering wheel and the steered vehicle wheels.
The electric motor is controlled by a control value outputted by a control apparatus.
The control value is calculated by the control apparatus based on a current command value, which is a control target value, and a driving current that actually flows through the electric motor. An assist current command value, which is a target value related to the assist force, is calculated by the control apparatus based on the steering torque of the steering wheel. On the other hand, a compensation current command value, which is a target value related to the compensation force, is calculated by the control apparatus based on the steering wheel angular velocity. The steering wheel angular velocity is estimated based on the terminal-to-terminal voltage of the electric motor.
The control apparatus always keeps determining whether there is an abnormality in the terminal-to-terminal voltage of the electric motor. Conventionally, the control apparatus conventionally discontinues outputting control values when an abnormality is detected in the terminal-to-terminal voltage. If the compensation current command value is computed based on abnormal terminal-to-terminal voltage, the control values may be invalid. The control apparatus stops the output of the control value to prevent invalid control values from being outputted. Control of the electric motor with invalid control values causes various drawbacks such as different steering feelings when the steering wheel is rotated leftward and rightward, and an unstable operation of the steering wheel when the vehicle is driving at a high speed. However, if the output of the control values is stopped, the generation of the assist force is discontinued, which, in turn, requires a greater force to manipulate the steering wheel on the part of the driver.
Accordingly, it is an objective of the present invention to provide an apparatus and method for controlling an electric power steering system, which apparatus and method prevent the steering feeling from being degraded when there is an abnormality in the terminal-to-terminal voltage of a motor, while continuing the generation of assist force with the motor.
To achieve the above objective, the present invention provides a control apparatus of an electric power steering system located between a steering wheel and a steered vehicle wheel. The electric power steering system includes a motor. The motor generates assist force for reducing steering force of the steering wheel and compensation force for compensating for movement of the steering wheel. The apparatus includes first calculation means, second calculation means, control means, and detection means. The first calculation means calculates an assist current command value that corresponds to the assist force based on the steering torque of the steering wheel. The second calculation means estimates the angular velocity of the steering wheel based on a terminal-to-terminal voltage of the motor and calculates a compensation current command value that corresponds to the compensation force based on the estimated steering wheel angular velocity. The control means controls the motor based on the sum of the assist current command value and the compensation current command value. The detection means detects an abnormality of a terminal voltage of the motor. When an abnormality of the terminal voltage is detected, the second calculation means substitutes a predetermined fixed value for the steering wheel angular velocity and then calculates the compensation current command value.
The present invention further provides a control method of an electric power steering system located between a steering wheel and a steered vehicle wheel. The electric power steering system includes a motor. The motor generates assist force for reducing steering force of the steering wheel and compensation force for compensating for movement of the steering wheel. The method includes: calculating an assist current command value that corresponds to the assist force based on the steering torque of the steering wheel; estimating the angular velocity of the steering wheel based on a terminal-to-terminal voltage of the motor, and calculating a compensation current command value that corresponds to the compensation force based on the estimated steering wheel angular velocity; controlling the motor based on the sum of the assist current command value and the compensation current command value; detecting an abnormality of a terminal voltage of the motor; and substituting a predetermined fixed value for the steering wheel angular velocity when an abnormality of the terminal voltage is detected.
Other aspects and advantages of the invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.